Recently I find that when I click on a link to a Web page, in Mail, it opens Safari on "uk.search.yahoo.com", not showing the page linked. There is just the empty search box and a blank page. However if I carefully click the edge line of the search box, not easy to position correctly, a list of ten previously visited Web pages appears below the search box. In that list is the page that I want and I have to click that to bring up the page. Before Yahoo started to appear I just clicked the link in Mail and I was quickly on the Web page required.
How do I stop Yahoo appearing? Can't see anything relevant in System Prefs or Mail Prefs.
Using iMac OS 10.13.6 High Sierra, Mail v 11.5, Safari v 12.0.3
On checking Safari prefs again, I found an extension called Search Manager. I uninstalled that and find that I am back to normal when clicking links! That’s a relief!
There are an awful number of sites now which you may go to for a legitimate download but when you get there you are presented with a large, often green, download button. In small text beside it which you are meant to miss it tells you you are about to download Mac Keeper or some other “nasty” - perhaps Search Manager. (Your desired download will be hidden away somewhere with a much smaller button!)